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TheMechanics of Earthquakes and Faulting
2nd Edition
Our understanding of earthquakes and faulting processes has developedsignificantly since publication of the first edition of this book in 1990. This revisededition has therefore been thoroughly up-datedwhilstmaintaining anddeveloping the twomajor themes of the first edition.The first of these themes is the intimate connection between fault and
earthquakemechanics, including fault scaling laws, the nature of faultpopulations, and how these result from the processes of fault growth andinteraction. This has lead to a fuller appreciation of how faulting and earthquakesare actually different timescalemanifestations of the same dynamical system. Thesecondmajor theme is the central role of the rate-state friction laws in earthquakemechanics. It is nowunderstood that these friction laws not only govern theearthquake instability itself, but also result in a gamut of other earthquakephenomena including seismic coupling and decoupling, pre- and post-seismicdeformation, earthquake triggering, and the relative insensitivity of earthquake totransients such as earth tides. Thus friction laws provide a unifying frameworkwithinwhich awide range of faulting phenomena can be interpreted.With the inclusion of two chapters which explain brittle fracture and rock
friction fromfirst principles, this book is written at a level whichwill appeal toscientists from a variety of disciplines for whoma complete understanding offaulting and earthquakes is the ultimate goal. Graduate students and researchscientists in the fields of seismology, physics, geology, geodesy, and rockmechanicswill greatly benefit from this book.
Christopher Scholz experienced his first earthquake in California at the age ofnine and has been asking questions about faulting and earthquakes ever since. Heis now Professor of Earth Science and AppliedMathematics at the Lamont-DohertyEarth Observatory, Columbia University, where he uses laboratory experiments,theory, and field-based techniques to study brittle deformation processes thatoccur in the outermost layer of the Earth. In addition to over 160 papers in theprimary literature, Professor Scholz is also the author of Fieldwork: a geologist’smemoir of the Kalahari (1997) and coeditor of Fractals in Geophysics (1989).
This publication is in copyright. Subject to statutory exceptionand to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press.
First published 2002First paperback edition 1990Reprinted 1992, 1994, 1997, 2000Second edition 2002Seventh printing 2012
A catalogue record for this publication is available from the British Library
Library of Congress Cataloguing in Publication dataScholz, C. H. (Christopher H.)The mechanics of earthquakes and faulting / Christopher H. Scholz. – 2nd ed. p. cm.Includes bibliographical references and index.ISBN 0-521-65223-5 (hardback) – ISBN 0-521-65540-4 (paperback)1. Seismology. 2. Faults (Geology) I. Title.QE534.2 .S37 2002551.22–dc21 2001043297
isbn 978-0-521-65223-0 Hardbackisbn 978-0-521-65540-8 Paperback
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It has now been more than thirty years since the publication of E. M.Anderson’s The Dynamics of Faulting and C. F. Richter’s Elementary Seismology.Several generations of earth scientists were raised on these texts. Althoughthese books are still well worth reading today for their excellent descrip-tions of faults and earthquakes, the mechanical principles they espousedare now well understood by the undergraduate student at the second orthird year. In the meantime a great deal has been learned about these sub-jects, and the two topics, faulting and earthquakes, described in thosebooks have merged into one broader field, as earthquakes have been moreclearly understood to be one manifestation of faulting. During this periodof rapid progress there has not been a single book written that adequatelyfills the gap left by these two classics. As a result it has become increasinglydifficult for the student or active researcher in this area to obtain an overallgrasp of the subject that is both up-to-date and comprehensive and that isbased firmly on fundamental mechanical principles. This book has beenwritten to fill this need.Not least among the difficulties facing the researcher in this field is the
interdisciplinary nature of the subject. For historical reasons earthquakesare considered to be the province of the seismologist and the study of faultsis that of the geologist. However, because earthquakes are a result of aninstability in faulting that is so pervasive that on many faults most slipoccurs during them, the interests of these two disciplines must necessarilybecome intertwined. Moreover, when considering the mechanics of theseprocesses the rock mechanicist also becomes involved, because the naturalphenomena are a consequence of thematerial properties of the rock and itssurfaces.It is a consequence of the way in which science is organized that the
scientist is trained by discipline, not by topic, and so interdisciplinary
subjects such as this one tend to be attacked in a piecemeal fashion fromthe vantage of the different specialties that find application in studying it.This is disadvantageous because progress is hindered by lack of communi-cation between the different disciplines, misunderstandings can abound,and different, sometimes conflicting, schools of thought can flourish in therelative isolation of separate fields. Workers in one field may be ignorant ofrelevant facts established in another, or, more likely, be unaware of theskein of evidence that weights the convictions of workers in another field.This leads not only to a neglect of some aspects in considering a question,but also to the quoting of results attributed to another field with greaterconfidence than workers in that field would themselves maintain. It is notenough to be aware, second-hand, of the contributions of another field –onemust know the basis, within the internal structure of the evidence andtools of that field, uponwhich that result ismaintained. Only then is one ina position to take the results of all the disciplines and place them, withtheir proper weight, in the correct position of the overall jigsaw puzzle.Because the literature on this topic has become both large and diverse, aguide is useful in this process, together with some unifying mechanicalprinciples that allow the contours of the forest to be seen from between thetrees.Although I have dabbled, to one degree or another, in the various differ-
ent disciplinary approaches to this problem and therefore have a rudimen-tary working knowledge of them, my own specialty is rock mechanics, andso this approach is the onemost emphasized in this book. Faults are treatedas shear cracks, the propagation of which may be understood through theapplication of fracture mechanics. The stability of this fault movement,which determines whether the faulting is seismic or aseismic, is deter-mined by the frictional constitutive law of the fault surface, and so that isthe second major theme applied throughout this treatment. The applica-tion of these principles to geology is not straightforward. One cannot actu-ally do a laboratory experiment that duplicates natural conditions.Laboratory studies can only be used to establish physical processes and vali-date theories. To apply the results of this work to natural phenomenarequires a conceptual jump, because of problems of scale and because boththe nature of thematerials and the physical conditions are not well known.In order to do this one must have constant recourse to geological and geo-physical observations and, working backwards, through these physical
principles determine the underlying cause of the behavior of faults. For thisreason, much of this book is taken up in describing observations of naturalcases.Because rockmechanics is not taught universally in earth science curric-
ula, the first two chapters present an account of brittle fracture and frictionof rock, beginning fromfirst principles. These chapters provide the basis forthe later discussion of geological phenomena. The subsequent chaptersassume a beginning graduate level understanding of the earth science dis-ciplines involved. In these chapters the results of geology, seismology, andgeodesy are presented, but the techniques employed by the various special-ties are not described at any length. The emphasis is on providing an overallunderstanding of a scientific topic rather than teaching a specific craft. Agoal was to describe each topic accurately, but at such a level that it could beunderstood byworkers in other fields.A book may be structured in many different ways. In this case, I found it
difficult to choose between organizing the book around the physical mech-anisms or around the natural phenomena in which they are manifested.The latter schemewould bemore familiar to the earth scientist, the formerto the mechanicist. Ultimately, I adopted a system arranged aroundmechanics, but which still retains many of the more familiar traditionalassociations. Because some mechanisms are important in a number of dif-ferent phenomena, which might otherwise be considered quite distant,and some earthquakes provide examples of several phenomena, there areoften more than two connections to other topics. Therefore, it was notalways possible to present the subject matter in a serial sequence. I conse-quently adopted a system of cross-referencing that allows the reader to tra-verse the book in alternative paths. I hope this system will be more helpfulthan confusing.When I first entered graduate school twenty-five years ago, most of the
material described in this book was not yet known. The first generation ofunderstanding, outlined in Anderson’s and Richter’s books, has been aug-mented by a second generation of mechanics, much more thorough andquantitative than the preceding. This has been a most productive era,which this book celebrates. I owemy own development to associations withmany people. My first mentor, W. F. Brace, set me on this path, and the wayhas been lit by many others since. I have also been a beneficiary of anenlightened system of scientific funding during this period, which has
allowedme to pursuemany interesting topics, often at no little expense. Forthis I particularly would like to thank the National Science Foundation, theUS Geological Survey, andNASA.Many have helped in the preparation of this book. In particular I
acknowledge the assistance of my editor, Peter-John Leone, Kazuko Nagao,who produced many of the illustrations, and those who have reviewedvarious parts of the manuscript: T.-F. Wong, W. Means, J. Logan, S. Das, P.Molnar, J. Boatwright, L. Sykes, D. Simpson, and C. Sammis. Particularthanks are due to T. C. Hanks, who offered many helpful comments on thetext, and who, over the course of a twenty-year association, has not failed topoint out my foibles. I dedicate the book to my wife, Yoshiko, who providedme with the stability in my personal life necessary for carrying out thistask.
When the first edition of this book was completed in 1989 the study ofearthquakes and faulting was still developing rapidly and has continued todo so in the intervening years. It thus seemed necessary, in order to keepthis work useful, that an extensively revised and updated new edition beprepared.Progress during these dozen years has not, of course, been uniform.
There have been rapid developments in some areas whereas others havebeen relatively static. As a result, some sections and chapters have beenextensively revised while others remain almost the same, undergoing onlyminor updating. A goal in this revision was to retain the same overalllength, and this has been largely successful. This necessitated the removalof material which in hindsight no longer seemed as vital as it once did orwhich had been superseded bymore recent results.The twomajor themes of the first edition have been further developed in
the interim. The first of these is the intimate connection between fault andearthquake mechanics. Fault mechanics in 1989 was still in a primitivestate, but rapid progress during the 1990s has brought the discovery of themain fault scaling laws, the nature of fault populations, and how theseresult from the processes of fault growth and interaction. This new knowl-edge of faultmechanics provides a fuller appreciation of faulting and earth-quakes as two aspects of the same dynamical system: the former itslong-timescale and the latter its short-timescale manifestation. One majordevelopment along these lines is the realization that neither faulting norearthquakes behave in an isolated manner but interact with other faults orearthquakes through their stress fields, sometimes stimulating the activityof neighboring faults, sometimes inhibiting it, the totality of such interac-tions resulting in the populations, of both faults and earthquakes, that areformed.
The secondmajor theme is the central role of the rate–state friction lawsin earthquake mechanics. These friction laws are now known to not onlyproduce the earthquake instability itself but to result in a gamut of otherearthquake phenomena: seismic coupling and decoupling, pre- and post-seismic phenomena, earthquake triggering, and the relative insensitivityof earthquakes to transients such as earth tides. Thus the friction lawsprovide a unifying strand for understanding the commonality ofmany phe-nomena previously thought to be disparate. Meanwhile the physics behindthese friction laws has become better understood, rendering them lessopaque than previously.The development and deployment of telemetered networks of broad-
band digital seismometers and of space based geodesy with GPS and InSARhas provided far more detailed descriptions of earthquakes and the earth-quake cycle than ever before. These observations have allowed for theirinversion for the internal kinematics of large earthquakes in well moni-tored regions like California as well as detailed descriptions of interseismicloading and postseismic relaxation, all of which has improved our under-standing of the underlying dynamics.Many people have helped in my preparation of this revised edition. I am
particularly indebted to Masao Nakatani, who offered many comments onshortcomings of the first edition and who helped me to better understandthe physical basis of the rate–state-variable friction laws.
In addition to many colleagues who have allowed me to reproduce theirgraphical material herein, I would like to thank the following copyrightholders who have graciously permitted the reproduction ofmaterial in thisbook.
ACADEMIC PRESS JAPAN, INC.From Mogi, K. (1985) Earthquake Prediction: Figure 7.7, his Figure 4.11; Figure7.8, his Figure 14.11; Figure 7.14, his Figure 13.16; Figure 7.16, his Figure15.12.From Advances in Geophysics: Plates 3, 4, and 5, King and Cocco (2000) 44: 1.
AMERICAN ASSOCIATION FOR THE ADVANCEMENT OFSCIENCESFrom Science: Figures 2.26 and 2.27, Raleigh et al. (1976) 191: 1230–7; Figures2.13 and 2.22, Shimamoto (1986) 231: 711–14; Figures 7.22 and 7.30, Scholz etal. (1973) 181: 803–9; Figures 7.21, Scholz (1978) 201: 441–2; Figure 4.17, Staff,USGS (1990) 247: 286.
AMERICAN GEOPHYSICAL UNIONFrom Journal of Geophysical Research: Figure 2.2, Brown and Scholz (1985) 90:5531; Figure 2.3b, Brown and Scholz (1985) 90: 12575; Figure 7.17, Das andScholz (1981) 86: 6039; Figure 5.3, Fitch and Scholz (1971) 76: 7260; Figure5.23, Schwartz and Coppersmith (1984) 89: 5681; Figure 6.5, Scholz (1980)85: 6174; Figures 7.31 and 7.32 Wesnousky et al. (1983) 88: 9331; Figure 2.5,Boitnott et al. (1992) 97: 8965; Figure 2.7, Wang and Scholz (1995) 100: 4243;Figure 2.11, Blanpied et al. (1995) 100: 13045; Figure 3.11, Vermilye andScholz (1998) 103: 12223; Figure 2.21, Blanpied et al. (1998) 103: 9691; Figure3.20 and 6.9, Cowie et al. (1993) 98: 17911; Figure 4.14, Hauksson et al. (1993)
98: 19835; Figure 5.5, Savage et al. (1999) 104: 4995; Figure 5.6, Savage (1995)100: 6339; Figure 5.7, Savage and Thatcher (1992) 97: 11117; Figure 5.9,Gilbert et al. (1994) 99: 975; Figure 5.10, Peltzer et al. (1998) 103: 30131; Figure5.11, Muir-Wood and King (1993) 98: 22035; Figure 5.36, Shaw (1995) 100:18239; Figures 6.15 and 6.16, Scholz and Campos (1995) 100: 22103; Figure7.28, Dodge et al. (1996) 101: 22371; Plate 8, Triep and Sykes (1997) 102: 9923.From Eos, Plate 6, Shen et al. (1997) 78: 477.From Earthquake Prediction, an International Review. M. Ewing Ser. 4 (1981).
Figure 4.28, Einarsson et al., p. 141; Figure 5.28, Sykes et al., p. 1784; Figures7.2 and 7.3, Ohtake et al., p. 53.From Earthquake Source Mechanics. AGU Geophys. Mono. 37 (1986): Figures
2.24 and 2.45, Ohynaka et al., p. 13; Figures 7.18 and 7.19, Deiterich, p. 37.From Geophysical Research Letters: Figure 3.35, Power et al. (1987) 14: 29;
Figure 4.8, Aki (1967) 72: 1217; Figure 4.28, Hudnut et al. (1989) 16: 199;Figures 5.13 and 5.25, Shimazaki andNakata (1980) 7: 279.From Tectonics: Figure 6.4, Byrne et al. (1988) 7: 833.
AMERICAN PHYSICAL SOCIET YFrom Physical Review B – Condensed Matter: Figure 2.20, Baumberger et al.(1999) 60: 3928.
A listing is given of the most important symbols in alphabetical order, firstin the Latin, then the Greek alphabets. The point of first appearance is givenin brackets, which refers to an equation unless otherwise indicated. Insome cases the same symbol is used for different meanings, and vice versa,as indicated, but the meaning is clear within the context used. Arbitraryconstants and very commonusages are not listed.